1. Field of the Invention
The present invention relates to a resonant pressure transducer and in particular a small transducer of this type (e.g. with a side length of a few millimeters) produced by microelectronics technology, particularly micromachining.
2. Description of The Related Art
Resonant pressure transducers are known and first of all a basic description will be given of the physical operating principle. Such sensors generally comprise a flexible, deformable diaphragm or membrane exposed to the ambient conditions, where the pressure to be measured prevails and which transmits, by means of a converter system, the force which it supports or withstands as a result of said pressure to a resonator system, which is vibrated by an exciter and whose resonator frequency is permanently measured. As this resonant frequency is a function of the force transmitted to the resonator system, it is clear that by measuring the said frequency, an analog magnitude representative of the pressure to be measured is obtained.
The systems used for exciting the resonator and for measuring the resonant frequency belong to numerous known types and involve known physical principles. Reference can e.g. be made to the use of elastoresistive gauges inserted in a WHEATSTONE bridge and fixed to the surface of the vibrating body constituting the resonator, the use of piezoelectric materials such as zinc oxide ZnO, the use of electrostatic forces between two coatings of a capacitor, whereof one is etched on the resonator or is constituted by the resonator, etc.
With reference to the attached FIG. 1, a description will be given of a pressure transducer with conversion by oscillating crystal in accordance with the prior art. FIG. 1 shows in a box or case 2 in which there is a vacuum, a system comprising a bellows or linked by an opening 6 in the wall of the case 2 with the atmosphere, whose pressure is to be measured. This bellows 4 which expands lengthwise to a varying extent as a function of said pressure firstly acts on a converter 8, which is a lever-shaped member articulated about a pivot 10. The end 12 of said converter 10 transmits the mechanical action undergone by the bellows 4 to an oscillator crystal 14, whose resonant frequency is measured. The resonant frequency is an analog value of the pressure which it is wished to measured.
However, such prior art devices suffer from a number of specific defects, both from a technical standpoint and from an economic standpoint. Details will now be given of these. With regards to the manufacturing costs of the apparatus, these are relatively high because mass production is difficult and instead such apparatuses are individually manufactured.
Moreover, the presently envisaged manufacturing processes do not make it possible to reduce the size of the transducers below a certain level (approximately 1 or more cm in exemplified manner), whereas increasingly frequent applications require small transducers, whereof the three dimensions are e.g. a few millimeters.
Moreover, the materials used for constructing such transducers are based on various technologies, whereas microelectronics is a "monolithic" art.
Finally, with regards to their technical performance characteristics, the known resonant pressure transducers, generally only accept frequency swings of 5 to 10% with respect to the resonant frequency of the resonator, which can e.g. be between 95 and 105 kHz. This feature, which is linked with the breaking stresses of the resonator, significantly limits the range of measurements possible using these transducers.